Incurious and Unread in this comment talked about a “modest perturbation” and positive and negative feedbacks:
I can’t help feel that Gaia is letting us down here. It seems that we keep on getting positive feedback effects on the climate rather than negative feedback.
That seems kind of odd. As I understand it, the climate has been fairly stable pre-AGW, which suggests that negative feedback effects dominated. We have what seems to me (probably in my ignorance) a modest perturbation and suddenly we are envisaging positive feedback and an uncontrollable excursion to a new Venusian equilibrium.
Is it just that it is the positive feedback stories which hit the headlines, or is there some underlying reason why there are more positive feedback than negative feedback effects?
First up, it’s forcings that change the temperature positively or negatively. Forcings are amplified by feedbacks, positive and negative. For a stable climate the net effect of forcings and feedbacks should be zero, they should cancel each other out.
The following graph shows the main radiative forcings:
CO2 is about equivalent to the net human forcing. The others cancel each other out.
As an aside, cloud effects seem to me to be a feedback rather than a forcing, and may turn out to be mildly positive. If so this is not good news.
Now have a look at this graph where CO2 is plotted against temperature for the last 800,000 years:
CO2 has varied from about 180 to 280 ppm to give us successive ice ages. Notice the current level of COI2 on the right. Distinctly immodest in that context.
Now have a look at this article which tells us that current CO2 levels are higher than they have been for 15 million years. That’s quite a perturbation in the space of about 150 years, although most of the increase has come since WW2.
This graph tells the story of the up-tick from the 1950s.
This one tells the story of the Holocene. Again distinctly immodest in recent times.
In his Idaho Testimony (Fig. 11) James Hansen tells us that in the Cenozoic era natural CO2 forcing was at the rate of 100 ppm/My (0.0001 per year) as against 2 per year now. Again somewhat immodest. A mere 20,000 times as much. This image shows us how such a mild rate of forcing has moved the temperatures over the last 65 million years.
Note where the temperature was 15 million years ago. Of course the wild swings of the last 3 million years have been primarily caused by orbital changes, a function of when the major ice sheets are engaged in the system in a particular temperature band, but that’s another story.
The effect of forcings and feedbacks on temperature is normally expressed in terms of watts par square metre. These graphs from Hansen at NASA show the current forcing at 3 watts per square metre of GHGs plotted against various phenomena:
If you follow the red line in the middle graph you get the impression that something big is going to happen. You can see in the third graph that the temperature beast is stirring. Remember that the long term feedbacks such as the ice sheet response take centuries to work their way through the system.
BTW, those graphs are from a few years back. I think the 3 watts forcing refers to short term Charney-type feedbacks.
The net forcings and feedbacks appear to translate into temperature at the rate of 0.75C for each watt per square metre. The direct effect of CO2 is actually quite small, but this is amplified through feedbacks. One of the main ones is water vapour. Warmer air carries more water vapour which in turn traps more radiation, which warms the air which… It must be a diminishing effect, because the end result is equilibrium rather than a runaway effect. One source gives the effect of water vapour as 1.8 watts per square metre of energy trapped for each extra C of temperature.
You will recall this image in the previous post:
So that image is based on the notion that the direct forcings give you 1C, amplified by short term feedbacks to 3C and longer term feedbacks to 6C. If along the way you trip off something like releasing methane from the permafrost, of fry the Amazon and other tropical forests, then be very afraid.
There is an account of forcings and climate sensitivity in Hansen’s Bjerknes Lecture. Go here and download the pdf from 20 December, 2008.
In that lecture he also addresses the possibility of a Venus effect. He thinks the Venus effect is possible if we burn all the coal in the ground. If we burn all the carbon available in tar sands he sees a Venus effect as a “dead certainty”.
So the take-out of this story is that what we’ve done in the last 100 years or so is anything but modest. A mere 100 ppm is equivalent to about 2 million years of natural change during the bulk of the Cenozoic. That’s apart from the Paleocene–Eocene Thermal Maximum (PETM) 55 myr ago, shown as a blip on Figure 5 above, where the temperature rose 6C over 20,000 years and then returned to normal for those times over 120,000 to 170,000 years. That’s fast in geologic time. But the perturbation we have caused and would cause under BAU in a few short centuries is thought to be of a similar order in terms of tonnes of carbon emitted into the atmosphere. That’s a blitzkrieg on the climate system.
In these circumstances Gaia is pushed rudely aside. But be warned, she will have the last word.
BTW Hansen thinks that Gaia as a factor that intentionally steers the system towards conditions suitable for life is basically rubbish. The earth system doesn’t care whether we turn into a Venus-like planet.
The is another perspective which says that conditions for life are really quite fragile and operate within a small band at the margin.
Without the greenhouse effect the average earth temperature would be about -18C, fluctuating madly on a daily basis. This is really quite warm on the average if you consider that’s 255 Kelvin. The greenhouse gases, a minuscule fraction of the atmosphere, which itself is like a coat of paint on a basket ball, give us an extra 32C or so and equalises the temperature across the 24 hours and the seasons.
Under present conditions we have a balance of about 340 watts per square metre in and roughly the same out. But a gap of a few watts has opened up because we are messing with a trace gas that represents about one 2500th of the atmosphere.
Some sceptics say that’s too small to have an effect. Is it? Every evening I take a one mg pill which helps me sleep at nights. It’s because I have a somewhat obsessive personality that leads my to write posts like this. In terms of my body weight that works out at one in 72 million. Seems to work OK.
A few watts on top of 340 adds a few degrees on top of 287 Kelvin and civilisation is kaputt!
So we best be careful how we mess with the earth system. Gaia if she exists doesn’t care what happens to us. We will be the authors of our own destiny.